A. Field of the Invention
This invention relates general to data collection and transmission systems, and more particularly to real time clock functions and timing methods for use with television audience measurement and broadcast program identification data collection systems.
B. Description of the Prior Art
An accurate time reference is often useful in various different data collection and monitoring systems. U.S. Pat. No. 3,651,471 issued to Haselwood et al., on Mar. 21, 1972 and related U.S. Pat. No. 3,742,463 issued to Haselwood et al., on Jun. 26, 1973 disclose a data storage and transmission system for monitoring via telephone a large number of remotely located television receivers or the like that utilizes an elapsed time counter for maintaining an elapsed time record stored as part of the recorded data changes.
U.S. Pat. No. 4,025,851, issued to Haselwood et al., on May. 24, 1977 discloses a system for automatically monitoring the programs broadcast by commercial television stations with a monitoring station having a clock for generating an elapsed time indicative signal and a central computer for providing a real time reference signal and for calculating the actual time of occurrence of a particular detected program change. The disclosed system, while effective for its intended purpose, requires considerable processing by the central computer to determine the real time reference data.
U.S. Pat. No. 4,677,466 issued to Lert, Jr. et al., on Jun. 30, 1987 discloses a broadcast program identification system with a station monitoring unit having a real time clock or a relative time clock that is periodically reset from an accurate clock, for example, a timing signal transmitted by the National Bureau of Standards.